In iOS 12 Family Sharing there’s now a setting for Purchase Sharing with an associated Apple ID. Mine is set to my Apple Store ID which is historically distinct from my iCloud ID (many old timers have this unfixable issue). If you tap on this Apple ID it rings up a dialog that allows this to be changed (there’s a bug here — tapping on it doesn’t always work. I had to leave the screen and return to it to enable tap). When I tapped it switched the default to my iCloud Apple ID.

I believe this is a new control. It will be interesting to see what happens when I migrate other family devices that use this iTunes Store ID for purchasing.

At the moment only one Apple ID can be used, but this UI could support multiple Apple IDs. The screen also displays a payment method that cannot be changed, it’s presumably defined by Apple ID.

This is something to watch.

PS. The ten year history of this mess is one reason I recommend Spotify over Apple Music for families.